SMBus Compatibility With an I²C Device

Posted on February 23, 2013

Some naive guys – like me – think that SMBus and I²C are equal expect the bus level. But in the last days I run into a problem where an at24 based EEPROM connected via SMBus triggers a bus collision after a warm reboot. After some hours of debugging – even staring minutes at a oscilloscope showing SDD and SDC lines – I started to add some dump_stack() calls into i2c_core.c file form a recent linux kernel. And I found the bad bus transaction – i2c_probe_func_quick_read(..).

But whats exactly the problem here?

The SMBus specification describes a number of standardized
transactions. Each chip on the bus can support each given transaction
type or not. In practice, each slave chip only supports a subset of the
SMBus specification. The Linux i2c subsystem abuses this
transaction type for device detection purposes, because it is known to
give good results in practice, but ideally it shouldn’t do that.

There have been some devices known to lock up the SMBus on Quick
command with data bit = 1. In most cases
these were write-only devices, which didn’t expect a transaction
starting like a read (the SMBus specification says that a Quick command
with data bit = 1 is writing 1 bit to the slave, but the I2C
specification says this is a read transaction of length 0.)

This SMBus Quick command thing keeps causing trouble and confusing
people. Not much we can do though. If your slaves don’t like the quick
command, just don’t send that command to them.